I’m still looking… Ez filter does 25-30. Not muchroch, but it makes sense for them to go with the most chomur.
http://israel613.com/books/TOLAIM_WATER_NY.pdf
BTW, my rov told me 30 is best, and I personally am sure that this is the shito l’chumro, I’m just trying to find you a source you’ll be happy with. Let me know when you’re satisfied.